On Monday, a Pentagon spokesman said North Koreans had been killed, without giving a number, and a day later an unnamed US official said “hundreds” had been killed or wounded.
The BBC has not independently verified these claims.
The North Korean troops, who have no previous combat experience, are believed to have spent their first few weeks in Russia training and acting in a support role.
The casualties are believed to have occurred in Russia’s Kursk region, where Ukrainian forces are defending a small territory captured in a surprise attack last August.
On Saturday, Ukrainian President Volodymyr Zelensky said Russia had begun deploying “significant numbers” of North Korean troops in the Kursk offensive.
They are not believed to be deployed in Ukraine itself, where Russian troops have been advancing into eastern Ukraine in recent months.
